Digital Modes: the Future of Amateur Radio? An introduction to PSK31, MT63, and Hellschreiber John DeGood NU3E Trenton Computer Festival Sun 7 May 2000 1 PSK31 • Invented by Peter Martinez, G3PLX • First PC soundcard version 26 Dec 1998 • Intended for live keyboard-to-keyboard QSO • Uses varicode character coding for 50 wpm • Easy to use and monitor • Gives very good copy under low Eb/No numbers and is thus suitable for QRP 2 PSK31 (continued) • Instead of using FSK or on/off keying, uses BPSK or QPSK with a Viterbi decoder • Is available for free for many platforms, including Windows with soundcard • Uses advanced DSP and narrow bandwidth (31.25 Hz) techniques • Tx duty cycle is 50% idle, 90% maximum • The greatest activity is around 14070.15 3 Bandwidth BPSK QPSK Spectra obtained with EvmSpec (from PSK31 homepage) 4 PSK31 Operation • BPSK is generally used for calling CQ and routine operation • QPSK gives much better performance with fading and flutter • QPSK has an 800 msec one-way delay, or 1.6 sec round-trip • PSK31 requires a synth or stable VFO rig – BPSK tuning needs to be within 8 Hz – QPSK tuning needs to be within 4 Hz 5 MT63 • Developed by Patwel Jolocha SP9VRC • Encodes information using 63 modulated tones • Sounds unusual, like a roaring noise • No connection process, as in AMTOR, Packet, or PACTOR • Outstanding performance when conditions are both weak and unstable. 6 MT63 (continued) • Spreads signal in time (several seconds) and space (500-2000 Hz) • Forward Error Correction (7-bit ASCII encoded into 64 bits using a Walsh function) can result in 100% copy even if 25% of a character is obliterated • Clumsy (slow) operation due to FEC delay • Very aggressive: causes interference to other modes, but itself is little affected by other modes 7 MT63 Characteristics Bandwidth Audio Range Symbol Rate Char Rate Interleave/char 500 Hz 500-1000 Hz 5 baud 5 char/sec 6.4 or 12.8 sec 1000 Hz 500-1500 Hz 10 baud 10 char/sec 3.2 or 6.4 sec 2000 Hz 500-2500 Hz 20 baud 20 char/sec 1.6 or 3.2 sec 8 100% Copy in Presence of Complex Fading with MT63 10 Watts on 14 MHz 50 Watts on 3.5 MHz 9 Hellschreiber (Hell Writing) • Patented by Rudolf Hell in 1929 • Used by the German Condor Legion during the Spanish Civil War (1933). During WWII, Hellschreiber was widely used for field portable military communications. • Still used today in the original format • Visually readable mode • Most DX on 14063 kHz - 14070 kHz 10 Feld-Hell (Field Hell) • Feld-Hell characters are sent as a series of dots at 122.5 pixels/sec, using a CW transmitter or by sending tones to an SSB transmitter. • Black dots are represented by a CW dot (key down), and white spaces by a space as long as one dot (key up). • Low duty-cycle: ~22% 11 The Siemens & Halske A2 Feldfernschreiber (1944) 12 Feld-Hell 7x7 Character Set 13 Feld-Hell Timing • Each character takes 400ms. Since there are 49 pixels per character, each pixel is 8.163ms long. The effective baud rate is 1/8.163 ms = 122.5 baud, and the throughput is 2.5 characters/sec, or about 25 WPM. • Raised cosine filtering results in 2 * 122.5 = 245 Hz bandwidth. 14 The First IZ8BLY Feld-Hell QSO January 1, 1999 15 PSK-Hell • Recently invented by Murray ZL1BPU and implemented by Nino IZ8BLY. • Uses Differential Phase Shift Keying, which relies on detection of a change in phase. One state (black) is defined as no change in phase from one dot to the next, while the other state (white) is defined as a reversal of the carrier phase from the previous dot. 16 PSK-Hell (continued) • Matrix is only 7 x 6, seven columns of six dots (42 dots). This reduction in dots per column allows a lower baud rate (105 baud) for the same column rate and text speed as Feld-Hell. • High Tx duty-cycle: ~90% 17 MT-Hell (Multi-Tone Hell) • Each row of dots or pixels in the character is at a different frequency, with quite different (much relaxed) timing, and thus is transmitted in the frequency domain. • Since columns are always sequential, there is no notion of synchronism in MT-Hell. • Reception of MT-Hell requires a technique called the Fast Fourier Transform to convert the signal back into a human readable form. 18 C/MT-Hell • C/MT-Hell or Concurrent MT-Hell uses many tones (seven or more), which are often transmitted at the same time. • C/MT-Hell has the following potential advantages: – Speed, since several dots can be sent at the same time – Better readability, since more dots provide better resolution – Better looking text, since the characters need not slope and many fonts can be used 19 S/MT-Hell • S/MT-Hell or Sequential MT-Hell uses only a few tones, typically five or seven, but never more than one at a time. • S/MT-Hell advantages: – Weak signal performance, since all the transmitter power is applied to a single dot – High efficiency: since the transmitter need not be linear, Class C transmitters can be used – Very simple transmitter requirements - easily adapted to QRP and LF transmitters – Simple signal generation with a PC, even via 20 the PC speaker or a Hamcom interface Appearance of Hell Modes • Feld-Hell text is usually very sharp and may lean slightly to the right, and is always double printed. • Sequential MT-HELL text leans strongly to the right, and may have a dotty (pixellated) appearance. • Concurrent tone MT-HELL has very detailed characters, and is always upright (unless the user has chosen an italic font!) 21 Summary • PSK31 in the USA has reached “critical mass”: there are signals on HF, especially 20 meters, at almost any time • MT63 sees limited activity. Its use of wide bandwidth is controversial. • Hellschreiber is a popular mode in Europe, but there is limited activity in the USA • PSK (and to a lesser extent, Hell) are great for QRP and restricted antenna installations due to great performance with low Eb/No. 22 Amateur Radio Digital Mode Information Resources • Official PSK31 Homepage – http://www.kender.es/~edu/psk31.html • Fuzzy Modes (and MT63) Web Site – http://www.qsl.net/zl1bpu/ • Digipan PSK31 Software – http://members.home.com/hteller/digipan/ • IZ8BLY Hellschreiber and MT63 Software – http://space.tin.it/computer/aporcino/ • Slides of This Presentation – http://www.nerc.com/~jdegood/tcf2000.pdf 23.